In my boat there are two fishboxes on either side in the cockpit area. If I put tanks along the sides where the rod racks are, then they sit about half way across on the fishbox lid.

Has anyone had any success in splitting the lid lengthwise and using maybe piano hinge so that it opens only half of the lid and you can drop in fish or extra gear or tanks without having to remove the side tanks?

Seems like a great idea to me. Although I would consider a stronger hinge than a piano hinge. You don't want it caving in on you. Not to mention a marine SS piano hinge, if you can find one, would likely be pretty expensive.

I've also used aluminum hinges in the boats since they are a lot cheaper. Another possibility might be to have the tanks above the hatch an inch or so, so the hatch could slide open from under the tanks. You could use a piece of starboard, trex, or wood to support the bottom of the tanks with enough clearance to slide the hatch out from under the tanks. Not sure if it would work with your configuration, but worth a look.

What about putting a "stiffener" on the underside edge of the cut, maybe some 1/4 or 1/2" inch of that material that's stiffer than Starboard and lighter, forgot name and then running a cross member under the hinged+stiffener that can be removed for full box access.